The abundance of different flower colors is one of the pleasant aspects of life that enriches human minds and hearts. It is expected to increase food production to meet future population increase by the means of accelerating the growth of plants through symbiosis with microorganisms, or by increasing the number of nitrogen-fixing leguminous bacteria, thus improving the plant productivity as a result of increasing the content of nitrogen in the soil. Elimination or reduction of the use of agricultural chemicals is also desirable to achieve more environmentally friendly agriculture, and this requires improvement of the soil by the above-mentioned biological means, as well as higher resistance of plants against microbial infection. Another desired goal is to obtain plants with high protective functions against ultraviolet rays as a means of protecting the plants from the destruction of the ozone layer.
“Flavonoid” is a general term for a group of compounds with a C6-C3-C6 carbon skeleton, and they are widely distributed throughout plant cells. Flavonoids are known to have such functions as attracting insects and other pollinators, protecting plant from ultraviolet rays, and participating in interaction with soil microorganisms (BioEssays, 16 (1994), Koes at al., p. 123; Trends in Plant Science, 1 (1997), Shirley, B. W., p. 377).
Of flavonoids, flavone plays an important role in interaction of plants with microorganisms, especially in legumes, where they participate in the initial steps of the symbiosis with leguminous bacteria (Plant Cell, 7 (1995), Dixon and Paiva, p. 1085; Annu. Rev. Phytopathol., 33 (1995), Spaink, p. 345). Flavones in petals play a role in recognition by insects and act as copigments which form complexes with anthocyanins. (Gendai Kagaku, (May, 1998), Honda and Saito, p. 25; Prog. Chem. Org. Natl. Prod., 52 (1987), Goto, T., p. 114). It is known that when flavone forms a complex with anthocyanin, the absorption maximum of the anthocyanin shifts toward the longer wavelength, i.e. toward blue.
The biosynthesis pathways for flavonoids have been widely studied (Plant Cell, 7 (1995), Holton and Cornish, p. 1071), and the genes for all of the enzymes involved in the biosynthesis of anthocyanidin 3-glucoside and flavonol, for example, have been isolated. However, the genes involved in the biosynthesis of flavones have not yet been isolated. The enzymes that synthesize flavones include those belonging to the dioxygenase family that depends on 2-oxoglutaric acid (flavone synthase I) and monooxygenase enzymes belonging to the cytochrome P450 family (flavone synthase II). These groups of enzymes are completely different enzymes with no structural homology.
It has been reported that in parsley, 2-oxoglutaric acid-dependent dioxygenase catalyzes a reaction which produces apigenin, a flavone, from naringenin, a flavanone (Z. Naturforsch., 36c (1981), Britsch et al., p. 742; Arch. Biochem. Biophys., 282 (1990), Britsch, p. 152). The other type, flavone synthase II, is known to exist in snapdragon (Z. Naturforsch., 36c (1981), Stotz and Forkmann, p. 737) and soybean (Z. Naturforsch., 42c (1987), Kochs and Grisebach, p. 343; Planta, 171 (1987), Kochs et al., p. 519). A correlation has been recently reported between a gene locus and flavone synthase II activity in the petals of gerbera (Phytochemistry, 49 (1998), Martens and Forkmann, p. 1953). However, there are no reports that the genes for these flavone synthases I and II were isolated or that flavone synthase II was highly purified.
The properties of a cytochrome P450 protein, which had licodione-synthesizing activity that was induced when cultured cells of licorice (Glycyrrhiza echinata) were treated with an elicitor, were investigated. The protein is believed to catalyze the hydroxylation of 2-position of liquiritigenin which is a 5-deoxyflavanone, followed by non-enzymatic hemiacetal ring opening to produce licodione (Plant Physiol., 105 (1994), Otani et al., p. 1427). For cloning of licodione synthase, a cDNA library was prepared from elicitor-treated Glycyrrhiza cultured cells, and 8 gene fragments encoding cytochrome P450 were cloned (Plant Science, 126 (1997), Akashi et al., p. 39).
From these fragments there were obtained two different full-length cDNA sequences, each encoding a cytochrome P450, which had been unknown until that time. Specifically, they were CYPGe-3 (cytochrome P450 No. CYP81E1) and CYPGe-5 (cytochrome P450 No. CYP93B1, hereinafter indicated as CYP93B1) (Plant Physiol., 115 (1997), Akashi et al., p. 1288). By further expressing the CYP93B1 cDNA in a system using cultured insect cells, the protein derived from the gene was shown to catalyze the reaction synthesizing licodione from liquiritigenin, a flavanone, and 2-hydroxynaringenin from naringenin, also a flavanone.
2-Hydroxynaringenin was converted to apigenin, a flavone, by acid treatment with 10% hydrochloric acid (room temperature, 2 hours). Also, eriodictyol was converted to luteolin, a flavone, by reacting eriodictyol with microsomes of CYP93B1-expressing yeast followed by acid treatment. It was therefore demonstrated that the cytochrome P450 gene encodes the function of flavanone 2-hydroxylase activity (FEBS Lett., 431 (1998), Akashi et al., p. 287). Here, production of apigenin from naringenin required CYP93B1 as well as another unknown enzyme, so that it was concluded that a total of two enzymes were necessary.
However, no genes have yet been identified for enzymes with activity of synthesizing flavones (such as apigenin) directly from flavanones (such as naringenin) without acid treatment. Thus, despite the fact that flavones have numerous functions in plants, no techniques have yet been reported for controlling their biosynthesis in plants, and improving the biofunctions in which flavones are involved, such as flower color. The discovery of an enzyme which by itself can accomplish synthesis of flavones from flavanones and acquisition of its gene, and introduction of such a gene into plants, would be more practical and industrially applicable than the introduction into a plant of genes for two enzymes involved in the synthesis of flavones from flavanones.